Yeah, Canon L Glass should hold up at 4K . It handles up to 18mp (lot's of photographers complain about it at higher resolutions because it's not as sharp as Zeiss, etc) and 4K Full Frame video is a little over 12mp.
Cheaper glass, no no haha.
The C300 is a sweet camera no doubt. As far as C300 vs Scarlet, really depends on what you're doing. Shooting tons of footage for Broadcast tv or Web purposes and have to edit and turn around asap, C300 wins. It'll also more likely fail less in the field. RED is a lot harder to use. HDRx files are massive, and 4K is massive. But dang does it look good and give you tons of data to work with. If you have lot's of time on set and can do lot's of takes and have time to let your computer chew on footage, and especially if you're doing a theater release or 35mm transfer of the final product, the Scarlet is probably better.
No perfect camera out there. Pros & Cons with all. Shoot with what you like, suits your purposes and what you have access to!